1. Field of the Invention
The present invention relates to a chucking structure for chucking an end mill having a relatively small diameter and used, for example, for deep milling of a die.
2. Description of the Related Art
FIG. 1 shows a conventional end mill chucking structure. As shown in FIG. 1, a straight hole 1c is formed in a front cylindrical portion 1b of a holder body 1a of a tool holder 1. An end mill 2 includes an end mill body 2a, which, in turn, includes a round-rod-shaped shank portion 2b. A rear portion of the shank portion 2b of the end mill 2 is fitted into the straight hole 1c formed in the tool holder 1. A plurality of side lock members 3a and 3b are screwed into the front cylindrical portion 1b of the tool holder 1 from the outer circumferential surface thereof so as to press the fitted rear portion of the shank portion 2b against the wall surface of the straight hole 1c, thereby fixedly attaching the end mill 2 to the tool holder 1.
In the case of the conventional end mill 2 shown in FIG. 1, a relatively long front portion of the shank portion 2b projects frontward from the tool holder. Thus, the shank portion 2b fails to maintain sufficient rigidity, resulting in an impairment in cutting performance and involving vibration, such as pitching, during cutting.
FIG. 2 shows another conventional end mill chucking structure. An end mill 2 includes an end mill body 2a of carbide, which, in turn, includes a shank portion 2b shorter than that shown in FIG. 1. An end portion of the shank portion 2b is shrink-fitted into a straight hole 1c formed in a tool holder 1. The outer circumferential surface of the end portion of the shank portion 2b is gripped by means of the wall surface of the straight hole 1c formed in the tool holder 1; i.e., single-surface restraint is established so as to fixedly attach the end mill 2 to the tool holder 1.
In the case of the conventional shrink-fitted chucking as shown in FIG. 2, the tool holder 1 is heated, and then the shank portion 2b of the end mill 2 is fitted to the tool holder 1. When the shank portion 2b is to be removed from the tool holder 1, the entire tool holder 1, to which the end mill 2 is attached, is heated so as to be thermally expanded. Then, the shank portion 2b is removed from the tool holder 1. Accordingly, the material of the tool holder 1 has a greater coefficient of thermal expansion than does the material of the end mill 2; i,e., the tool holder 1 and the end mill 2 must be formed of metals of different coefficients of thermal expansion.
In this connection, the applicant of the present invention filed Japanese Patent Application No. 10-354175 on Dec. 14, 1998 in relation to a shank-integrated end mill. As is disclosed in the patent application, a front portion of a holder body of an end mill holder is formed into a taper portion which tapers down frontward. A small-diameter hole is formed coaxially in the taper portion in such a manner as to extend rearward from a front end. A rear portion of a shank portion of an end mill body is formed into a small-diameter portion. The small-diameter portion of the end mill body is shrink-fitted into the small-diameter hole formed in the holder body, such that a shoulder located at the front end of the small-diameter portion abuts the front end face of the taper portion of the holder body. Cutting means is provided at a front end portion of the end mill body. In the shank-integrated end mill, the shank portion of the end mill body is rendered short to thereby decrease the cost of the end mill. Further, the outer circumferential surface of the small-diameter portion of the shank portion of the end mill body is gripped by means of the wall surface of the small-diameter hole formed in the end mill holder, and the entire face of the shoulder of the shank portion abuts the front end face of the end mill holder; i.e., two-surface restraint is established so as to fixedly attach the end mill body to the holder body. Thus, the rigidity of the end mill becomes considerably high, so that vibration hardly occurs during cutting, resulting in good cutting performance.
FIG. 3 shows still another conventional end mill chucking structure using a collet. The collet includes a taper shank portion 4A, which is fitted into a taper hole 1A formed in a tool holder 1, and a taper arbor portion 4B, which extends over a predetermined length from the larger-diameter end of the taper shank portion 4A coaxially with and in opposition to the taper shank portion 4A while tapering down. A straight hole 4C having a predetermined length and diameter is formed coaxially in the arbor portion 4B in such a manner as to extend from the front end thereof toward the taper shank portion 4A. The arbor portion 4B is heated to a temperature of about 300xc2x0 C. to thereby be thermally expanded. A straight shank portion 2b of an end mill 2 is fitted into the thermally expanded straight hole 4C. The end mill 2 is thus chucked in the collet 4 through shrink fit.
The taper shank portion 4A of the collet 4 to which the end mill 2 is shrink-fitted is inserted into the taper hole 1A formed in the tool holder 1. Subsequently, a draw bolt 5 is screwed into a female-threaded hole 4D formed in an end portion of the taper shank portion 4A so as to fixedly attach the collet 4 to the tool holder 1, thereby fixedly attaching the end mill 2 to the tool holder 1.
In the case of the conventional shrink-fitted chucking as shown in FIG. 3, the shank portion 2b of the end mill 2 and the hole 4C, into which the shank portion 2b is shrink-fitted, are both straight. Accordingly, when the shank portion 2b of the end mill 2 is to be shrink-fitted into the straight hole 4C formed in the collet 4 or when the end mill 2 is to be removed from the collet 4 through application of heat, the arbor portion 4B of the collet 4 must be heated to a temperature of 300xc2x0 C. or higher for smooth shrink-fitting or removal. When shrink-fitting or removal is repeated frequently for replacement of the end mills 2, the material of the arbor portion 4B undergoes a change in physical properties due to frequent application of heat, even when the collet 4 is made of special steel having excellent coefficient of thermal expansion and strength. As a result, the collet 4 fails to chuck the end mill 2 reliably.
Since the end mill 2 shrink-fitted to the collet 4 is restrained merely by means of the shank portion 2b in contact with the wall surface of the strait hole 4C formed in the collet 4, the rigidity of an end portion of the end mill 2 if projecting from the collet 4 becomes relatively low, resulting in a strong likelihood of subtle vibration during cutting.
An object of the present invention is to solve the above problems involved in the conventional end mill chucking structure, and to provide attend mill chucking structure enabling press-fitting, shrink-fitting, or removal of an end mill through application of heat at relatively low temperature in order to prevent a change in physical properties of material and capable of improving the rigidity of the end mill.
To achieve the above object, the present invention provides an end mill chucking structure comprising an end mill holder, which comprises a holder body, and an end mill, which in turn comprises an end mill body and cutting means provided at a front end portion of the end mill body. The holder body has a taper hole formed in a front portion thereof. The taper hole tapers down rearward and extends rearward from a front end of the holder body. Herein, the term xe2x80x9cfrontxe2x80x9d refers to a cutting-end side of the end mill attached to the end mill holder with respect to the axial direction of the end mill holder, and the term xe2x80x9crearxe2x80x9d refers to the side opposite the front side. A rear portion of the end mill body is formed into a shank portion tapering down rearward. The rear end face of the front portion of the end mill body extends outward from the outer circumferential surface of the shank portion. The shank portion of the end mill body is press-fitted or shrink-fitted into the taper hole formed in the holder body through application of heat at a temperature lower than a conventional shrink-fitting temperature such that the rear end face of the front portion of the end mill body abuts the front end face of the holder body, to thereby fixedly attach the end mill body to the holder body. According to the present invention, heat for press-fitting or shrink-fitting may be applied to the holder body at a relatively low temperature, and the end mill body can be easily attached to or detached from the holder body. Since the shank portion of the end mill can be shortened, the end mill can be rendered inexpensive. The outer circumferential surface of the shank portion of the end mill body is gripped by means of the wall surface of the taper hole formed in the holder body, and the rear end face of the front portion of the end mill body abuts the front end face of the holder body; i.e., two-surface restraint is established, thereby maintaining considerably high rigidity of the end mill and preventing vibration of the end mill during cutting to thereby yield good cutting performance.
Preferably, the rear end face of the front portion of the end mill body extends perpendicularly from the outer circumferential surface of the shank portion. Thus, vibration of the end mill during cutting can be prevented more effectively.
Preferably, the front portion of the end mill body is formed into a relatively long taper portion which tapers down frontward. The rear end of the taper portion has a diameter smaller than that of the front end face of the holder body. Thus, stable chucking is established between the end mill and the end mill holder. In the case of wear of a cutting edge, a throwaway tip may be replaced without removal of the end mill from the holder body. Material for the end mill is not required to have a coefficient of thermal expansion different from that of material for the end mill holder. The end mill may be made of cemented carbide or, for example, HSS, which has a coefficient of thermal expansion substantially identical to that of material for the end mill holder.
Preferably, the rear end face of the front portion of the end mill body is caused to abut the front end face of the holder body by means of a draw bolt accommodated within the front portion of the holder body, to thereby fixedly attach the end mill body to the holder body. Through employment of the draw bolt, the rigidity of the end mill is improved, so that cutting can be performed more reliably. Alternatively, female threads are formed on a wall surface of the taper hole formed in the holder body to thereby form a female-threaded portion; male threads are formed on the outer circumferential surface of a rear portion of the shank portion of the end mill body to thereby form a male-threaded portion; and the male-threaded portion is screwed into the female-threaded portion to thereby fixedly attach the end mill body to the holder body. As compared to the case of employment of the draw bolt, the rigidity of the end mill is impaired slightly; however, the holder body and the end mill can be manufactured more easily.
Preferably, a groove is formed on the wall surface of the taper hole formed in the holder body, or a small hole is formed in the holder body, so as to discharge fluid, such as coolant, therethrough and frontward from the holder body. Preferably, a discharge angle modification member for directing fluid to be discharged from the holder body is disposed at a front end portion of the small hole which extends axially in the holder body so as to discharge the fluid therethrough. Through discharge of fluid from the holder body, a workpiece can be cooled, or adhesion of chips to the workpiece or end mill can be prevented, as needed, during cutting.
The present invention further provides an end mill chucking structure comprising an adapter having a shank portion and an arbor portion. A taper hole of a predetermined depth is formed coaxially in the arbor portion and extends from the front end of the arbor portion toward the shank portion. An end mill has a mill body portion and a taper shank portion extending coaxially from the rear end of the mill body portion. The taper shank portion is press-fitted or shrink-fitted into or removed through application of heat from the taper hole formed in the arbor portion of the adapter. Thus, the end mill can be shrink-fitted into or removed from the adapter with ease, through application of heat at such a low temperature as not to cause a change in physical properties of adapter material. Also, an end mill having a small diameter can be chucked easily.
According to the present invention, the rear end face of the mill body portion projects outward from the taper shank portion in a flange-like manner so as to serve as a contact face, and abuts the front end face of the arbor portion of the adapter upon fitting of the taper shank portion into the taper hole. Thus, the end mill is restrained by means of two surfaces; specifically, the outer circumferential surface of the taper shank portion and the contact face, thereby improving the rigidity of the end mill and preventing vibration of the end mill during cutting to thereby yield good cutting performance.
Preferably, a front end portion of the adapterxe2x80x94into which the end mill is press-fitted or shrink-fitted and from which the end mill is removed through application of heatxe2x80x94is made of steel having a relatively high coefficient of thermal expansion, thereby further lowering the temperature at which heat is applied to the front end portion of the adapter for press-fitting or shrink-fitting the end mill thereinto or for removing therefrom.
Preferably, a passage is formed in the adapter and end mill so as to supply fluid, such as coolant, therethrough to a cutting point. Through supply of coolant, a workpiece can be cooled, or adhesion of chips to the workpiece can be prevented, during cutting, thereby yielding good cutting performance free of cutting scorch or scratches.
Preferably, the shank portion of the adapter is tapered, thereby not requiring use of a taper collet and decreasing run-out of the end mill.
Preferably, a locking mechanism is employed in order to prevent rotation of the end mill fitted to the adapter. When a small-diameter end mill is used, a gripping force of a front end portion of the adapter decreases. Specifically, as the front-end diameter of the adapter decreases, the wall thickness of the front end portion of the adapter decreases. As a result, during cutting, the shank portion of the end mill slips within the hole formed in the front end portion of the adapter due to expansion of the hole. The locking mechanism prevents such slippage of the end mill, thereby providing a highly accurate end mill chucking structure which does not involve an increase in run-out of the end mill.